This invention relates generally to a method and apparatus for the measurement of the dimensions of an object, and more particularly to a non-contact system to generate measurement data representative of planar sections of an object and contour maps of an object (which data may be used as an input to suitable process control apparatus), and to a spatially coded pattern for use with such system. The invention will be described primarily in connection with using electromagnetic radiation to obtain measurement data representing planar sections (profiles) of the external surfaces of logs so as to compute the three-dimensional surface profile of each individual log for the purpose of adjusting sawing equipment in saw mills (for example, to optimize the quantity or value of the lumber produced). However, the invention is also applicable to measurement of other objects, particularly where rapid and accurate shape determination is necessary. Such measurements may be made both as to objects' external surfaces and also as to internal interfaces (e.g. imaging and measurement of internal organs for medical purposes), the latter when suitable penetrating radiation is reflected from such internal interfaces and detectable (as reflected) by a suitable receiver. Further, the invention, while described as using electromagnetic radiation, is applicable to measurements using other forms of radiation, such as sound or particles, so long as reflection occurs from either an external or internal interface, and so long as a pattern divisible into distinguishable pattern elements as herein described may be applied to such radiation and detected by a suitable receiver.
The simplest non-contact automatic method commonly used to determine the shapes of logs is known in the prior art as shadow scanning. The log moves past a row of beams of light and the cross-sectional width of the log is determined by measuring the shadow cast by the log on an array of sensors on the other side of the log, which sensors are lined up with the projected light beams. Beams of light must be applied from several directions and sensed by a corresponding set of sensor arrays to obtain even a rough profile. The shadow method cannot measure or even detect concave features such as hole in the log. It measures the outer envelope of the profile of the log.
Other methods known in the prior art for determining the shape of an object without contact depend on the principle of triangulation, which has been known historically prior to the present century. The application of this principle can be illustrated by considering a single beam of light transmitted in a known direction in space from a known location at an object being measured. Some suitably selected form of receiving system positioned so as to view the object from a direction different from the direction at which the light was transmitted detects the direction from the receiving system at which the reflection from the projected light spot appears on the object being measured. The distance between the transmitter and the receiver is known and fixed. Hence two angles (determined from the transmitting and receiving directions) and one side of a triangle (the distance between the transmitter and the receiver) are determined, and thus the location of the spot on the object relative to the measuring apparatus is easily calculated.
To use triangulation to measure the shape of an object (rather than merely to measure the coordinates of a single point), many spots in a raster pattern or the like would have to be determined. This could be done by projecting a pattern of beams simultaneously, by sweeping one beam over the surface of the object in a suitable scanning pattern in a continuous motion, or by projecting a sequence of beams, one at a time, at different points on the object being measured (this technique is often referred to as "time multiplexing"). Simultaneous projection, as taught by the prior art, is not able to reliably measure irregular surfaces because identification of a particular spot with a particular transmitted beam becomes uncertain or ambiguous if any spots are obscured. In some cases, the ambiguity or uncertainty could be overcome by the use of a reference spot whose coordinates are known, thereby enabling the operator to establish a correspondence between at least one transmitted beam and one detected spot. However, ambiguity or uncertainty would remain a problem for other spots in the pattern, even if the reference spot were located unambiguously, as identification of spots other than the reference spot depends on the assumption that no spots between the reference spot and the spots to be identified are obscured. While beam sweeping and time multiplexing do not entail the foregoing ambiguity problem, both are subject to the problem that the accurate instantaneous measurement of the profile of an object such as a log moving rapidly is difficult, by reason of the need for adequate computing time required to determine each profile. For example, in a typical saw mill, logs move at 500 feet per minute, so that to obtain profiles of, say, 1" apart (axially) on the log requires that each scan take less than 10 milliseconds.
An alternative surface profile measurement apparatus taught in Leong, U.S. Pat. No. 4,937,445, granted on 26 Jun. 1990, that is alleged to achieve unique identification of detected spots with transmitted beams, uses a small number of beams, so that within a limited depth of range, the spot from each beam will be observed within a limited region on the imaging device. However, this implies that increasing the number of beams to increase resolution of surface features decreases the range of depths that can be measured. Further, accurate knowledge of the direction of each beam in the Leong technique is critical, making frequent calibration necessary.
An alternative taught in Corby, U.S. Pat. No. 4,687,325, granted on 18 Aug. 1987, is to project onto the scanned object a time-separated series of different complete-scan patterns of beams so that identification of the pattern of spots on the scanned object can be used to identify beams uniquely with detected spots. Triangulation is used to obtain the spatial coordinates of the spots. Corby requires the sequential projection of a series of mutually differing patterns of beams, and so suffers from the same problem from which beam sweeping and time multiplexing suffer, namely the inability to determine the instantaneous profile of a rapidly moving object. Furthermore, complexity arises in Corby from the need to transmit a plurality of different patterns in time sequence.
For the foregoing reasons, it can be readily understood that the problem of measuring at a distance the surface profiles of irregular objects moving rapidly along a production line (say) is not solved satisfactorily by the known art. A satisfactory measuring apparatus should:
(a) have either (i) the capability to make very fast (snapshot) measurements of the profile of the object so that as the object moves past the measuring apparatus, the surface contour of the entire object can be built up as a series of profiles; or (ii) the capability to make a measurement of the entire surface contour of the object at one time;
(b) have the ability to cope with failure to receive portions of the transmitted pattern (due to irregularity of surface features of the object or to the occlusion of portions of the object by intervening spurious objects);
(c) be compact, rugged, with a minimum of moving parts;
(d) not require frequent calibration; and
(e) have sufficient resolution and depth of field to measure accurately irregular objects such as logs.
The prior art teaches that a multiplicity of discrete beams (a pattern) projected simultaneously onto the object to be measured from different angles is needed to satisfy the requirements set out above for the rapid measurement of the complete surface profile of the object. However, the beam patterns taught in the prior art are not satisfactory as they do not enable reliable measurements to be made in situations that can occur in a sawmill and in other scanning situations, namely that the received signal may not represent the entirety of the transmitted scanning beam. There are various reasons why this may happen. The log may be smaller than the transmitted scanning beam. Irregularities on the surface of the object being scanned (e.g., bumps on the log) may occlude a portion of the log's surface such that the scanning beam does not reach the surface in question, or the bump may occlude the light reflected from the portion in question. Further, a log is carried by a conveyor, and sometimes the scan intersects a portion of the conveyor instead of the log, the log's surface being occluded by such conveyor portion. Consequently, the reflected light signal may be unreliable; portions of it may have to be rejected. Furthermore, if only a portion of the total scanned beam is received reliably by the detector, it may not readily be possible (within the teaching of the prior art) to correlate the received portion with any particular part of the log or other object being scanned. If the received signal cannot reliably be correlated with a particular portion of the object being scanned, then the received signal may be useless to the purpose at hand.